During the flight testing of missiles, vibration sensors (transducers) measure the vibrating motion at selected locations on the missile. These measurements are generally transmitted to a ground station for subsequent analysis. Sample rates for the power spectral density (PSD) vibration data typically range between 10,000 to 50,000 samples per second. When the samples are digitally quantized to 8 bits, the telemetry system must allocate 80,000 to 400,000 bits per second of telemetry capacity to each vibration sensor. The required telemetry capacity becomes excessive when more than three transducers are used. A typical flight test requires twenty or more sensors at different test points on the missile, however, to provide adequate details about vibration modes. Therefore, a data compression procedure which retains adequate resolution of vibrational frequencies is needed.
Since transmission of the uncompressed vibration data would require telemetry equipment that would impose severe weight penalties upon the missile, several alternatives to compress the data have been suggested. For example, in U.S. Pat. No. 3,094,692, Westneat et al. suggests statistical telemetering of high frequency vibration data from a missile after on-board processing to extract PSD features, correlation functions, and features of the amplitude probability distribution. Significant information is lost or sacrificed by assuming that the statistical data adequately characterizes all the sources of vibrations. The entire signal is analyzed to extract features associated with high frequency vibrations that can be characterized as random functions. Raw data, particularly low frequency data, is not transmitted.
Another similar method analyzes the entire raw signal to determine the amount of signal power that resides in each of a predetermined set of frequency bands within the total bandwidth. A common set of bands is a logarithmic progression of the center frequency, and is achieved with a suitable analog or digital filter. The filters provide a bandwidth at each band which is proportional to the frequency. One-third and 1/6 octave bands are commonly used.
This compression method often fails to provide sufficient resolution for accurate determination of the important frequency components of the spectral components of the vibration data. The compression is too coarse. Maximum band separation at each filter in the bandwidth makes interpolation between bands difficult and crude. Thus, the frequency can only be determined to within an accuracy related to the bandwidth of the particular band. Furthermore, if two important frequencies are within a signal band both contribute to the vibration, but only the combined power will be measured. In this way, important information can be lost or masked.